Republican candidate Mitt Romney and President Barack Obama sat at a table next to each other.

The debate, held in Boca Raton, Florida, comes just two weeks before Election Day and as polls show a tight race.

Barack Obama and Mitt Romney are tangling over foreign policy with both candidates still looking for a breakout from a deadlocked White House campaign with just two weeks to run.

Polls show Mr Obama with a small advantage about which candidate is best prepared to handle US foreign policy in chaotic world. Mr Romney will do his best in the 90-minute debate to minimise the president's accomplishments and win the support of the small slice of undecided voters among the millions watching.

The former Massachusetts governor has been hitting Mr Obama hard on the administration's changing explanations of what happened in last month's attack on the US Consulate in Benghazi, Libya, where militants killed four Americans, including US Ambassador Chris Stevens.

The Syria violence, Iran-Israel tensions, China, terrorism and the war winding down in Afghanistan were expected to come up in the final debate.

As the November 6 vote approaches, 41 of the 50 US states are essentially already decided, and the candidates are fighting over the remaining nine battleground states, including the critical Ohio and Florida.

The battleground states assume outsized importance because the presidency is decided in state-by-state contests, not by a national popular vote. The system can lead to a candidate winning the popular vote but losing the presidency, as former vice president Al Gore did in 2000.

With early and absentee voting already under way in many battleground states, including Ohio, North Carolina and Iowa, tight poll results indicate that the race could be decided by which campaign is best at getting supporters to the voting booth.